build: Enable LTO on-by-default for CMake and LTO off-by-default for Bazel

[openroad-ci]

CMake

• There were two LINK_TIME_OPTIMIZATION settings in CMakeLists.txt, which disables LTO by default.
• Fixed it to make LTO on-by-default for CMake.

Bazel

• LTO is off-by-default for Bazel. To enable LTO for Bazel, we should provide --config=opt. (e.g., bazel build //:openroad --config=opt).
• The Bazel configuration to enable LTO only for -c opt and disable LTO for all other compile modes (e.g., -c fastbuild, -c dbg, etc) requires modification of all BUILD files.
• Thus, rather than modifying the all BUILD files, implemented the LTO-on under --config=opt option.
• Added the following notification message when --config=opt is not given.

Issue

Fixes #9050

Previous PRs

#9254
#9069

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[gemini-code-assist]

Code Review

This pull request modifies the build configurations for both CMake and Bazel to control Link-Time Optimization (LTO). For CMake, LTO is now enabled by default, which should improve performance for release builds. For Bazel, LTO is now explicitly controlled by a --config=opt flag, and a helpful notification is added to inform users when it's not being used. The changes for Bazel are well-implemented. However, I have a concern regarding the CMake change. The removed code contained a comment about LTO interfering with Qt. Enabling LTO by default might break GUI builds. I've added a specific comment with a suggestion on how to address this potential issue.

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CMakeLists.txt (24-29)

The removed code block included a comment stating that LTO interferes with Qt (# Interfers with Qt so off by default.). By enabling LTO by default, this could potentially break GUI builds which use Qt.

Has this interference been resolved? If not, it would be safer to conditionally disable LTO when BUILD_GUI is enabled to avoid breaking existing functionality. For example, you could add logic to disable LINK_TIME_OPTIMIZATION if BUILD_GUI is true.

[jhkim-pii]

By enabling LTO by default, this could potentially break GUI builds which use Qt. Has this interference been resolved?

Yes. LTO is disabled for GUI module.